San Francisco needs to better preserve its below-market-rate housing while building more such units to meet The City’s goal of a creating one-third of new homes for low- to moderate-income buyers, according to a first-of-its-kind report released Tuesday night.

The Housing Balance Report, compiled by the Planning Department with data from 2005 to 2014, gives The City a cumulative housing balance score of 21 percent.

That level is calculated by comparing the number of units withdrawn from protection, including rent control, with the number of new homes built, along with acquisitions and housing rehabilitation projects.

The report stems from an ordinance approved by the Board of Supervisors in April requiring city planners to monitor housing units amid a major development boom in recent years. City planners will update the report twice a year to determine whether 33 percent of new housing units are below market rate, as urged by a nonbinding proposition passed by voters in 2014.

“We are doing quite well in terms of the production of new housing in The City … but what is causing the overall balance number to be lower is the loss of rent-controlled units,” said John Rahaim, director of the Planning Department.

Between 2005 and 2014, some 21,870 units were added to The City’s housing stock, of which 6,560 were below market rate, according to the report. More than half of that housing — nearly 12,940 units, including more than 3,300 at below market rate — was built in District 6, which includes the Tenderloin, South of Market, Civic Center, South Beach, Mission Bay, Rincon Hill and Treasure Island/Yerba Buena Island neighborhoods.

District 10 — which includes the Bayview-Hunters Point, Potrero Hill, Dogpatch and Visitacion Valley neighborhoods — saw the second highest number with 2,470 new units, including 1,060 at below market rate.

Additionally, the highest number of no-fault evictions occurred in districts 8 (Castro, Noe Valley and Glen Park), 9 (Mission and Bernal Heights) and 6, at 844, 688 and 641, respectively.

Of the 6,828 residential units from projects that have received site permits as of the first quarter of this year, nearly 24 percent are earmarked for below market rate, according to the report.

For advocates of below-market-rate housing, the report’s data came as no surprise.

“People in the affordable housing movement across the country have been saying for a long time preservation is important,” said Don Falk, chief executive officer of the nonprofit developer Tenderloin Neighborhood Development Corp. “This just amplifies the importance of that point.”

Falk said one solution could involve developing nonprofit-owned housing to keep such residential units at permanently below market rate.

Peter Cohen, co-director of the Council of Community Housing Organizations, noted a number of efforts are in the works that aim to increase or preserve the housing stock, including various measures expected on the November ballot.

“We need to increase the new [below-market-rate] housing development and we need to decrease the loss of existing rental housing, and that’s the way to make the math work,” Cohen said.

Meanwhile, Mayor Ed Lee on Wednesday announced The City’s purchase of a parcel at 490 South Van Ness Ave. to build at least 72 permanently below-market-rate family rental housing units. That site marks the fourth new multifamily below-market-rate housing development in the Mission to arise this year.